This invention relates to measuring and testing and more particularly to an improved press cycle monitor for measuring the peak forces exerted in repetitive cycles of a stamping press, a drill press or a similar production machine.
Many production machines are operated in a repetitive or cyclic manner for performing operations on successive workpieces. In stamping presses, for example, a piece of stock material is positioned between two open dies and the dies are then closed and a high force is exerted therebetween to cut and shape the stock material into a desired configuration. After shaping, the dies are opened, the shaped workpiece is ejected from the dies and additional stock material is advanced and the cycle is repeated. Under normal production, the machine operates automatically and does not require the services of an operator. However, problems may occur at any time requiring the immediate attention of an operator. For example, stock may fail to feed into the dies, or a stamped part may fail to eject from the dies after they are opened, or the dies may jam or stick due to a malfunction such as a misalignment. If the difficulty is not promptly attended to, a large number of defective workpieces may be produced or the press may be damaged. Therefore, it is desirable to have a monitor to indicate when a press or other type of cyclic manufacturing machine is not operating in a desired manner.
It is known that the operation of many repetitive manufacturing machines may be monitored by monitoring the pressure exerted by the machine during each successive cycle. U.S. Pat. No. 3,257,652, for example, discloses a monitor for a hydraulically operated stamping press in which the hydraulic pressure during each cycle is measured and displayed as a pressure-time curve on an oscilloscope. The pressure versus time curve for a normal operating cycle is marked on a mask which is positioned over the oscilloscope. If the curve for any cycle deviates from the curve marked on the mask, an operator may take corrective measures. However, this system requires designing a separate mask for each different operating condition. If the dies are changed to manufacture a different product, the mask also must be changed since the pressure-time curve will change. Or, if the speed of the press is changed, a new mask must be made since the pressure-time curve will be compressed if the speed of the press in increased or will be expanded if the speed of the press is decreased.
There has been difficulty in the past in adapting press monitors for operating automatically with different press cycle times and also for operating for measuring and indicating the highest peak of the press pressure curve where the pressure curve may have several peaks in a cycle. For example, as a press is actuated during a cycle, there may be a single pressure curve or the pressure curve may oscillate and may have multiple peaks. The highest pressure peak may be any one of the multiple peaks. If several lower level peaks follow the highest peak, there is a tendency for the press monitor to read and indicate the last or most recent peak even though this is not indicative of the highest press pressure. Problems also may occur with the press monitor sensing and indicating low level noise or other spurious signals. If the press cycle monitor is adapted to provide an alarm in the event of a low signal, the false reading may cause an alarm condition and the press may be shut down even though the press is operating correctly.